One of two 6BZ6 tubes in my TR4CW has a black, burnt look to the glass on the inside and reception is poor on all bands. Do tubes have this appearance when they fail? Is the silver coating normal in the top of the tube? Thanks. Ted KB8ZKL

There is probably nothing wrong with your tube. The black burn mark on the inside of a tube is caused by a "getter". After the tube is evacuated of air, it passes through a loop of wire where a pulse of RF ignites the getter containing magnesium powder. This small burning inside the tube insures that all air is completely gone thereby creating a "hard vacuum". This is part of the manufacuring process. Don't worry about it.

As tubes age the inside of the glass will tend to take on a cloudiness. The internal haze can be any shade from white through a medium to charcoal grey depending on the tube materials and the operating conditions, as in plate voltage and current. A tube shield can influence this as shielded tubes tend to run hotter.

There are exceptions to this, as I recall RCA made plenty of 6V6's that were coated grey inside but had a circle of clear glass on top so you could see the filament. I'd replace the tube you described as a first step in troubleshooting the receiver issue. The hazy glass doesn't make the tube inherently bad, it just means the tube has seen plenty of operating hours and is probably below normal efficiency.

Now, for a little tube lore...

In a darkened room, it's not unusual to note a faint bluish corona effect around the grid(s) and inside the plate. This is particularly true of the larger output tubes that run at higher voltages. If you see a bluish corona outside the plate and up against the glass, the tube is going gassy. The exception to this are the gas-thyratron voltage regulators like the 0A2 which glow magenta-purplish when working properly.

The silver spot inside the glass is called the 'getter' and is a mercury compound intended to attract and hold oxygen atoms since no vacuum is perfect. If the getter turns a reddish-brown, it has picked up enough oxygen to become mercuric oxide. That tube is very gassy and should be replaced on sight. If the getter is white, the glass is cracked somewhere.

(every so often you'll see a piece on eBay that has a tube or two with a 'white nose'... They're cracked)

On rare occassions you'll come across a mercury-vapor rectifier and notice small beads of mercury inside the tube. When replacing, try to shake the larger beads down into the base of the tube by gentle tapping and run the tube for a good 15 minutes or better with only filament voltage applied. This heats the tube enough to 'vaporize' the mercury into a semi-conductive gas. Mercury-vapor tubes should always be pre-heated before applying voltage to the plates, usually in the range of 5-10 minutes.

Finally, tube plates are made from various metals and coatings and in some cases (like an 8080 regulator) can be made of graphite. The plate should never glow a dull red. If it does, you're pulling far too much plate current and this can be caused by a leaky capacitor across a rectifier, mis-tuned finals in a transmitter, or a short in the load. Nothing (other than rolling off the bench onto a concrete floor) ruins a tube faster than pulling too much current.

The 6BZ6 was common as a VHF or IF amp in TV sets and, if I recall correctly, they usually had clear glass. There were tubes of similar types made with a grey band around the middle of the cylinder, but that was obviously intentional and wouldn't be described as 'blackish' or burnt looking. Since the discolored 6BZ6 is likely in the front end of the receiver section, I'd replace it...

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